The present invention relates generally to radar receivers and specifically to an envelope detector.
The portion of the radar receiver that extracts the modulation from the carrier is the detector. One particular type of detector is the envelope detector which recognizes the presence of the desired signal on the basis of the amplitude of the carrier envelope. The radar receivers derive the in-phase (I) and quadrature (Q) components of the received echo signals which is used in the envelope detection function.
Modern radar systems operate at certain signal-to-noise ratios in order to achieve a specified probability of detection without exceeding a specified probability of false alarm. High false alarm rate in the detection process can result if the noise level is lowered to achieve greater dynamic range, due to the bias added by the output of the envelope detector.
The task of improving the envelope detection function is alleviated, to some degree, by the prior art techniques given in the following patents: which are incorporated herein by reference
U.S. Pat. No. 4,298,942 issued to Lee on Nov. 3, 1981; PA0 U S. Pat. No. 3,599,208 issued to Nelson on Aug. 10, 1971; PA0 U.S. Pat. No. 4,231,005 issued to Taylor, Jr. on Oct. 28, 1980; PA0 U.S Pat. Nos. 4,168,500 and 4,168,501 issued to Brassaw on Sept. 18, 1979; PA0 U.S. Pat. No. 3,829,671 issued to Gathright et al on Aug. 13, 1974; and PA0 U.S. Pat. No. 3,858,036 issued to Lunsford on Dec. 31, 1974.
Lee discloses a nonlinear amplitude detector whose output is based upon a Binomial expansion of the inphase and quadrature signal components. The smaller signal component is squared and divided by the large magnitude signal component. In the patent the quotient from this operation is multiplied by a coefficient that is selected to provide desired amplitude error deviation characteristics. Signal amplitude is then determined by summing this product with the larger magnitude signal component.
While the Lee patent is very instructive in signal amplitude detection from the signal's inphase and quadrature components, the emphasis of Lee is reduction of the processing requirement in signal approximation. What remains required is improved error performance strictly tailored towards elimination of bias of envelope detectors while improving a radar system's dynamic range.
The remaining patents point in the correct direction of signal approximation by performing functions of I and Q pairs but, like Lee, are inappropriate for this specific engineering task. The two Brassaw patents show arrangements for averaging inphase and quadrature signals and then dividing them by a constant. In Nelson the magnitude of the quadrature error signal is used to control the gain or sensitivity of the monopulse receiver. Taylor, Jr. develops inphase and quadrature components in conjunction with a phase discrimination constant false alarm rate (CFAR) system. The Taylor, Jr. system includes a phase angle sensor for measuring the phase angle of the echo signal as one of at least eight predetermined phase angle representations with binary numbers in Gray format based on the relative magnitudes and polarities of the inphase and quadrature components of the echo signal. Both the Gathright et al and Lunsford patents are included as showing circuits for approximating the square root of the sum of I and Q squared signals in a radar.
For many years, a simple approximation has been used for performing the envelope detection function for quadrature pairs. The exact envelope detector has an output voltage given by (1). ##EQU1##
The approximation adds tne absolute magnitude of the larger of the quadrature pairs to one-half the absolute magnitude of the smaller. This algorithm is given in (2). ##EQU2##
Analysis has shown that an ideal implementation of this approximation results in negligible (about 0.1 dB) loss in the detection process in a radar system. In the digital mechanization, however, the divide-by-two for an odd number results in an output that is 0.5 Q (Q=quanta) low. Since the quantity, either I or Q, that is divided by two has a probability of 0.5 of being odd, the average output is low by a constant 0.25 Q. The CFAR (constant false alarm rate) circuitry typically averages a number of samples on noise from the envelope detector and multiplies by a constant, K, to provide threshold for detection. If the average value of noise from the actual envelope detector is N.sub.a, and the average value from the ideal approximate detector is N.sub.i, then the threshold, T, is calculated as shown in (3), EQU T.sub.a =K.sub.a N.sub.a K.sub.a (N.sub.i -0.25) (3)
whereas the desired threshold for a specific false alarm rate is given in (4), EQU T.sub.i =K.sub.i N.sub.i (4)
Since the ideal and actual thresholds should be equal, the relationship between K.sub.a and K.sub.i can be derived as shown in (5), EQU K.sub.i /K.sub.a =1-0.25/N.sub.i ( 5)
As evident from (5), the actual multiplier must be greater than the ideal multiplier and can provide a ccrrect threshold at only a single value of N.sub.i. Hence, the CFAR circuitry does not result in a constant false alarm rate when the amplitude of noise varies. As long as N.sub.i is large, the false alarm rate does not decrease greatly when N.sub.i increases, but increases drastically when N.sub.i is reduced. The problem, therefore is to eliminate the 0.25 Q bias in the actual envelope detector.
In the radar system for which this invention was conceived, the envelope detector also divides the output of the detector by two so that the output of this specific envelope detector is given in (6). ##EQU3##
By similar analysis, it can be shown that this mechanization increases the amount of bias in the actual envelope detector to 0.375 Q. Obviously, it is even more desireable to compensate for this bias.
In view of the foregoing discussion, it is apparent that there currently exists a need for compensating for the bias of radar receiver envelope detectors to avoid excessively high false alarm rates in the detection process when the noise level is lowered in the attempt to achieve greater dynamic range. The present invention is directed towards satisfying that need.